PZT piezoelectric ceramic containing Pb(Ti,Zr)O3 (hereafter referred to as “PZT”) as a primary component is typical piezoelectric ceramic used for a monolithic piezoelectric element, e.g., a piezoelectric actuator, a piezoelectric resonator, or a piezoelectric filter, in the public domain.
Furthermore, as for this type of monolithic piezoelectric element, those produced by laminating internal electrode layers formed from a Ag—Pd alloy and piezoelectric ceramic layers formed from PZT piezoelectric ceramic alternately and conducting co-firing have been used frequently. However, since Pd is expensive, research and development of monolithic piezoelectric elements in which inexpensive Cu is used as a material for internal electrodes has been conducted intensively.
For example, Patent Document 1 describes a monolithic piezoelectric element in which internal electrode layers contain Cu as a primary component and PZT piezoelectric ceramic is used.
This Patent Document 1 discloses a monolithic piezoelectric element provided with electrodes containing Cu as a primary component and piezoelectric ceramic, wherein the piezoelectric ceramic contains a perovskite type oxide, which is represented by a general formula ABO3 and in which the A site includes Pb and the B site includes Zr and Ti, as primary components, the B site includes an acceptor element composed of a divalent metal element and a donor element composed of a pentavalent metal element, and 0.42<a/b<0.5 is satisfied where the total number of moles of acceptor element is represented by a and the total number of moles of donor element is represented by b.
In the case where the electrodes containing Cu as a primary component and the piezoelectric ceramic are co-fired, Cu in the electrode is diffused into the piezoelectric ceramic, and Cu in the state of a Cu ion functions as a divalent acceptor element. Therefore, regarding the monolithic piezoelectric element in Patent Document 1, the B site of the piezoelectric ceramic composition is specified to include donors excessively in such a way that 0.42<a/b<0.5 is satisfied where the total number of moles of acceptor element is represented by a and the total number of moles of donor element is represented by b and, thereby, it is intended to suppress reduction in piezoelectric constant by canceling reduction in the average valence of B site due to diffusion of Cu.
Patent Document 1: International Patent Publication WO 2005/071769
However, in the case where the internal electrode material containing Cu as a primary component is used as in Patent Document 1, it is necessary that co-firing is conducted in a reducing atmosphere in order to suppress oxidation of Cu. Consequently, there is a problem in that a sufficiently high piezoelectric constant d33 cannot be obtained.
Furthermore, it is known that if the B site is allowed to include donors excessively as in Patent Document 1, the sinterability deteriorates. Therefore, in the case where the B site is allowed to include donors excessively, there is a need to conduct firing at relatively high temperatures. For example, regarding the monolithic piezoelectric element in Patent Document 1, firing is conducted while the firing temperature is set at 1,000° C. (refer to the paragraph No. [0048] in Patent Document 1).